1. Technical Field
This invention generally relates to coated polymer materials and, more particularly, to a resorptive polymer coat having a ceramic polymer blend which, when used individually or in combination, provides a highly tunable resorption time and a means of supplying bone forming elements.
2. Discussion
The repair of separated or dislocated bone fragments or segments following bone surgeries requires realignment of the separated or dislocated fragments or segments and subsequent secure fixation for promoting proper natural rejoinder of these bone fragments or segments. The presence of relative motion of the bone fragments or segments at a fracture or osteotomy location may result in irritation of the surrounding tissues, nonunion between the bone fragments, and an extension of the time of fracture healing. It is therefore desirable to accomplish as completely as possible an immobilization of the fracture or osteotomy site. This involves the relative fixation of affected bone segments relative to each other and in relation to the surrounding bone structure.
Known methods for providing fixation between adjacent bone portions have included the use of metallic plates of varying configurations, which are secured across osteotomies or fracture sites by metallic bone screws. These devices have been made of biocompatible metals and metal alloys, such as commercially pure titanium, stainless steel and cobalt chrome molybdenum. Other materials and devices, such as wires, intramedullary nails or externally fixed pins have also been used to reduce bone fracture mobility and to improve the relative position of adjacent segments. The aim of fixation of adjacent bone portions is to immobilize the fracture or osteotomy sites in order to promote localized bone growth in the natural repair of the separation.
The disadvantages associated with the use of metallic and metallic alloy devices relate to the possible undesirable cosmetic results associated with the protrusion of these devices above the bone surface, especially in locations directly beneath the skin, that is, without any intervening soft tissue for masking the implant devices from being noticed externally. As such, the only way to remove these implant devices involves revision surgery after the localized bone area has healed. In addition, metal and metallic alloy devices often should be removed from a pediatric patient so as to prevent growth restrictions. Another disadvantage associated with using metallic implants is the lack of load transfer. As the fracture site heals, load bearing capability should transfer from the implant to the surrounding tissue. This is possible only with resorbable materials.
The use of medical implant devices made from bioresorbable materials has been described in literature and these devices have the advantage of being absorbed by the body over a period of time so as to allow for bone or fibrous material to become repaired at a fracture or osteotomy site by growing into the space created between adjacent bone portions. Many bioresorbable materials have been suggested for use in fixation of adjacent bone portions. It was believed that these materials had to be extremely strong to fixate the bone portions over a relatively long period of time. This typically meant that the osteosynthesis plate had to be relatively thick and be made out of a high molecular weight oriented material such as poly L-lactic acid in which the molecular weight would exceed 250,000. See Pihlajamaki, H., et al., xe2x80x9cAbsorbable Pins of Self-Reinforced Poly-L-Lactic Acid for Fixation of Fractures and Osteotomies,xe2x80x9d Journal of Bone and Joint Surgery, v. 74-B, n. 6, p. 853-857, November 1992. In addition, it was believed that certain copolymers of glyceride and lactide were not appropriate for use in osteosynthesis plates because of a rapid loss of mechanical strength. Grijpma, D. W., et al., xe2x80x9cPoly (L-lactide) Crosslinked with Spiro-bis-dimethylenecarbonate,xe2x80x9d Polymer, v. 34, n. 7, 1993 at 1496.
While others suggest the use of non-reinforced materials, the molecular weight of the material had to be increased to maintain strength. In this regard, one author suggested using a non-oriented material having an average molecular weight of 106. See Bos, R .R .M., et al., xe2x80x9cLate Complications related to Bioresorbable Poly (L-Lactide) Plate-Osteosynthesesxe2x80x9d, Journal of Oral Maxillofacial Surgery, Supp. 3, 51(a) 1993 at 190. However, there were certain problems which were associated with these particular osteosynthesis plates. First, such osteosynthesis plates tended to have a relatively high degree of inflammatory response and therefore had to be removed from the patient. See, Bostman, O., xe2x80x9cCurrent Concepts Reviewxe2x80x94Absorbable Implants for the Fixation of Fractures,xe2x80x9d Journal of Bone and Joint Surgery, pp. 148-153, 1991. In addition, the osteosynthesis plates had to be made relatively thick so as to provide the requisite strength and resorption time, which tended to make the osteosynthesis plates have an unwanted cosmetic appearance when implanted.
A need therefore exists for a bioresorbable fastening device for bone fixation, such as an osteosynthesis plate, that is thin enough and of a suitable material to be resorbed over a desired period of time, yet is of sufficient strength to maintain relative bone fixation over the time period needed for the natural repair of fractures or osteotomies between adjacent bone portions. A need also exists for a bioresorbable osteosynthesis plate which has adaptable resorption rates, composition, and strength. A need further exists for such a bioresorbable device to allow for the formation of one or more additional fastener openings at one or more required precise locations during the surgical procedure.
In accordance with the teachings of the present invention, a bioresorbable material is disclosed having a ceramic and polymer coating. The ceramic and polymer coating increases the tailorability and tuneability of resorption rates and properties and allows for increases in implant design flexibility by virtue of its simplicity. Also as the coating comprises of a resorbable ceramic, it helps in bone integration and formation.
In one preferred embodiment, a resorptive material includes a polymer and ceramic coating to control the osseoconductive properties of the coating. A resorbable ceramic powder is deposited onto a substrate by use of a resorbable polymer binder. It is possible to use a combination of differing ceramic compositions as well as ceramic powder particle sizes to adjust resorption properties. Similarly, it is possible to use a combination of resorbable polymeric binders in different amounts to adjust resorption time.
In another preferred embodiment, a material having a biocompatible resorbable ceramic with biologically acceptable cations such as calcium, sodium, potassium and anions of phosphates in various oxidation states, carbonates, bicarbonates and sulfates including but not limited to calcium sodium phosphate, calcium sulfate, hydroxyapatite, calcium carbonate, tricalcium phosphate and octacalcium phosphate or a mixture of resorbable ceramics.
In yet another preferred embodiment, a method of forming a resorbable coating material onto a substrate is disclosed. The method includes the steps of forming a mixture of the polymer binder, a resorbable ceramic powder and a solvent. The mixture is disposed onto the substrate at a fixed thickness. The solvent is either extracted or evaporated off, leaving a coating of ceramic powder coupled to the substrate by a polymer binder.
Use of the present invention provides a substrate material coated with a resorbable layer that affects resorption rate. The coating material is formed of a resorbable polymer binder and resorbable ceramic materials. By adjusting the volume fraction of the ceramic, the thickness of the coating, the molecular weight of the binder and the composition of the binder, the resorption rate of the coating can be significantly slowed down. As a result, the aforementioned disadvantages associated with the currently available resorbable materials have been substantially reduced or eliminated.